1. Field of the Invention
The present invention relates generally to a bias balancing circuit, and more particularly, the bias balancing circuit of the invention is capable of balancing a bias and eliminating the effect caused by the offset voltage while switching input signal sources.
2. Description of the Prior Art
In electronic circuit applications, a steady voltage source and current source often play very important roles. An unsteady voltage source will easily mislead an electronic component into wrong decision or erroneous judgment in accordance with the logic. For example, according to the regulations of a digital circuit, an n-type metal-oxide-semiconductor field-effect transistor (MOSFET) is conducted if the input voltage at the gate electrode of n-type MOSFET is 3.3 V (namely, high level); n-type MOSFET is not conducted if the input voltage is 0 V (namely, low level). While the input voltage at the gate electrode of n-type MOSFET is unsteady, then n-type MOSFET will easily judge a voltage slightly lower than 3.3 V as a lower level and results in a disconnection of the electronic device and therefore, lead to a wrong motion of the entire electronic circuit.
Additionally, the influence of bias on a digital circuit has an unexpected outcome at the output end caused by the bias. For example, if the input voltage of an operational amplifier is an alternating current as a sine wave with amplitude of 3 V, the magnifying power is 4, and the output voltage is 12 V, ideally the bias should be 6 V as a target value. When the bias is unsteady and the amplifier is not precise enough, the operational amplifier may output irregular waves or cause the distortion of waves. That proves that a steady voltage source and current source play important roles in electronic circuit fields.
Please refer to FIG. 1. FIG. 1 is a functional block illustrating the conventional motor sound process wafer 1. When users are switching different signal sources (sound sources), there is a gain in motor sound process wafer 1 corresponding to each of different signal sources. Beforehand, a power supply is needed for the operational amplifier (for generating gain) in motor sound process wafer 1. Generally, a dual power supply operational amplifier is mostly adopted. Under a dual power supply, the output signal of the dual power supply operational amplifier changes to and fro on the origin of 0 V. There are operational amplifiers with single power supply. As for cars, the output voltage of a single battery is 12 V, which is difficult to afford a dual power supply. Moreover, a dual power supply operational amplifier is more expensive, which means that a single power supply operational amplifier is more suitable for motor sound process wafer 1. When using a single power supply operational amplifier, adding an extra power supply voltage which is half as much as the origin power supply voltage is necessary at the end without input signal (the former grounding end) and taking the extra voltage as a bias for the operational amplifier. After adding the extra bias mentioned above, the output waveform of the operational amplifier changes to and fro on the ½ Vcc (half as much as the power supply voltage). The main purpose of taking the input bias half as much as the operating voltage, is to get the maximum amplitude of output wave.
For ideal integrated circuit (IC) wafer, offset voltage is not expected while switching different input signal sources. However, offset voltage dose exist while switching different input signal sources of the general IC wafer, and the offset voltage often ranges from 1 mV to 10 mV. This situation may be caused by the leakage current of the cross-linked capacitance in front of the input buffer inside of the IC wafer, and also weather the circuit layout inside the IC wafer is appropriately operating in coordination or not. Generally, different input signal sources may couple with the input buffer of one-port amplifier or that of differential amplifier through the coupling capacitor. For example, while switching different input signal sources, gain stage may be switched from the input buffer of one-port amplifier to that of a differential amplifier. More particularly, because of the mismatch of the circuit layout of internal resistance inside the input buffer of the differential amplifier, the offset voltage is often greater than that of the input buffer of a one-port amplifier. If the amplified offset voltage goes through the gain stage to the volume stage, users will hear clearly the noise amplified by the gain stage while adjusting the volume.
When users switch the signal source then leading to the amplified gain, the offset voltage is amplified simultaneously. As for bias, it is very sensitive to the change of the voltage source. The fluctuation of the voltage source affects the change of bias. As everyone knows, bias is used for controlling electronic devices (such as transistor) to work on the region of users' requirement. So, while the bias is affected by the offset voltage to fluctuate, the working region of electronic devices changes with the unsteady bias. For example, when the target value of the bias drifts up, then the lower half of the output wave could be completely revealed but the upper half of the output wave would be cut off and could not be shown entirely. As illustrated in FIG. 1, the motor sound process wafer 1 has 28 pins. To eliminate the noise amplified by the gain stage while adjusting the volume, the capacitance 12 between pin 122 and pin 124 and the capacitance 14 between pin 142 and pin 144 are used to close off the bias to avoid the offset voltage affecting to the next stage (volume stage).
However, in the process of producing IC wafers, the package process occupies a large proportion of cost. If the number of pins could be diminished from 28 to 24 (diminish pin 122, pin 124, pin 142, and pin 144 as mentioned above), the cost of producing an IC wafer would be lessened. Therefore, how to diminish the capacitance 12 and the capacitance 14 while laying out the circuit, but still avoid the influence of offset voltage on bias, this is the main idea for low cost IC producing process.
Accordingly, the main aspect of the present invention is to provide a bias balancing circuit to balance the bias while switching input signal sources and eliminate the influence of offset voltage. By using the bias balancing circuit of the invention, the capacitances in the conventional IC wafer for separating the previous and the next bias could be replaced. Thus, not only the cost of the replaced capacitances could be avoided, but also the number of pins could be lessened to decrease the cost of the IC wafer considerably.